COALESCENCE OF SLIPPED JOINTS IN GRANITIC PLUTONS TO FORM THE WEST PINNACLE FAULT, SIERRA NEVADA, CALIFORNIA

The West Pinnacle fault is a N-striking zone of dextral shear in the Sierra Nevada batholith. Lockwood and Lydon (1975, Mt. Abbot quadrangle) mapped the fault as “intensely sheared mylonite” but, in current terminology, the fault mainly consists of foliated cataclasite and phyllonite that ranges in thickness to >100 m. We identified two distinctive mafic dikes that are dextrally offset ~0.5 km near the northern terminus of the fault, but the anatomy of the fault zone suggests that dextral offset greatly increases southward from the intersecting dikes. Fault-related deformation defines a highly asymmetric pattern that appears to reflect growth of the fault system. East of the fault, the Lamarck and Lake Edison plutons contain abundant steep E- to NE-striking joints. As much as several km from the fault, such joints commonly have accommodated from a few mm to several meters of sinistral offset and contain from <1 to ~20 cm of fault rock. The slipped joints are antithetic to the main fault and can be interpreted to have accommodated northward dextral shear by the bookshelf mechanism. The bulk shear strain magnitude, measured by summing slip over 100 - 500 m traverses, varies erratically but appears to increase westward toward the main fault. Within 1 km of the main fault, NE-striking faults increase in abundance and contain up to 1 m of cataclasite. The changes probably reflect increased bulk strain but sparseness of markers and sheer abundance of faults render the strain gradient difficult to quantify. In contrast, the Turret Peak granodiorite on the west side of the fault contains relatively sparse joints and few of these display evidence of significant slip. NE-striking slipped joints thus appear to have propagated southwestward into the eastern margin of the Turret Peak where they coalesced into a single N-striking fault zone. The field relations suggest two alternative, not mutually exclusive, reasons that the Turret Peak formed a propagation barrier that focused displacement onto a single fault: (1) the Turret Peak contains N-striking throughgoing joints that arrested propagation of NE-striking faults and collected their displacement; and (2) the Turret Peak contains abundant 1-2 cm Kspar phenocrysts which perhaps may yield a somewhat higher fracture toughness than the even-grained Lamarck and Edison granodiorites.